The Obama campaign used Clinton to buoy the Democratic convention, then had him star in the president’s post-convention ad buy — and not just because the former president remains immensely popular. As the campaign acknowledges, Clinton brings credibility to the connection between an Obama presidency and a strong economy, reinforcing the idea that there’s a straight line between Obama’s proposals and Clinton’s legacy of budget surpluses and middle class prosperity.

“I can’t recall a time when someone who’s not on the ballot has been this important,” said Paul Begala, the former Clinton strategist and current adviser to the pro-Obama super PAC Priorities USA Action.

First lady Michelle Obama’s approval ratings rival Clinton’s, but there’s no mistaking that Clinton is Obama’s most important surrogate, the one that he’s leaning on. For all the questions swirling about Mitt Romney’s personal appeal and the circular firing squad of Republicans raising doubts about his economic argument, Obama remains vulnerable on the economy and lacks Clinton’s natural ability to connect with people, despite the president’s high likability ratings.

Clinton — whose popularity has moved up steadily almost every year since the end of his scandal-ridden presidency — is a uniquely effective messenger.

“There’s a testimony that Clinton and only Clinton can give, which is: ‘I know how to build an economy that works, and this president is doing the right things,’” Begala said. “It allows the debate to be eight years of Democratic economics, which is exactly the stuff that President Obama is trying to enact, versus eight years of Republican economics, which is exactly the stuff that Romney wants to go back to.”

And the Obama campaign isn’t pretending otherwise. The Obama ad that featured a laid back Clinton chatting about helping the middle class — building up to “that’s what happened when I was president” — has already aired almost 16,000 times in its first month on the air, making it the most-run spot of the campaign, Bloomberg News reported Thursday.

The Obama campaign has been eager to use Clinton on additional stops after his post-convention swing through Florida, and the planning is underway for where those will be for after the New Hampshire trip next week. Clinton put his name on the latest reelection fundraising email. He‘ll be on the trail with Obama and is being used to promote the next small-donation raffle prize.